Referring to the relevant provisions of the International Atomic Energy Agency, in accordance with the degree of potential harm to human health and the environment, from high to low radioactive sources can be divided into Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ category.
Type I sources of radiation are extremely dangerous. In the absence of any protection, exposure to such sources a few minutes to 1 hour may cause death.
Type II radioactive sources are high-risk sources. In the absence of any protection, exposure to such sources for a few hours to a few days can lead to death.
Class III sources are medium risk sources. In the absence of any protection, exposure to such sources can cause permanent damage to people in a few hours, and exposure for a few days to a few weeks can cause death.
Type IV sources are low-risk sources. Basically will not cause permanent damage to people, but for a long time, close contact with these sources of radiation may cause temporary damage.
Type V radioactive sources are very low risk sources. Will not cause permanent damage to people.
Expanded:
Radiation sources can be categorized into alpha radiation sources, beta radiation sources, gamma radiation sources, and neutron sources according to the type of rays they emit. The rays emitted by radioactive sources have a certain amount of energy, which can damage cellular tissues and thus cause harm to the human body. When a person is exposed to a large amount of radiation, symptoms such as dizziness, loss of appetite, nausea, vomiting and other symptoms may occur, which can lead to serious damage to the organism, and may even lead to death; however, when a person is exposed to only a small amount of radiation, he or she generally will not have any uncomfortable symptoms, and will not harm his or her body.
References:
Baidu Encyclopedia - Radioactive Sources